Response of sorption processes of MCPA to the amount and origin of organicmatter in a long-term field experiment

Changes in farming practices over long times can affect the sorption behavi our of MCPA ((4-chloro-2-methylphenoxy)acetic acid). We studied the adsorpt ion-desorption mechanisms of MCPA on soil with Varied amounts and origins o f soil organic matter obtained from a long-term field experiment with vario us organic amendments. The origin of the soil organic matter seems to be cr ucial for the sorption behaviour of MCPA. Samples of soil amended with sewa ge sludge sorbed MCPA more strongly than the soil under any other treatment . Feat-amended soil was second followed by sail receiving animal manure, gr een manure, mineral fertilizer without N and the fallowed soil. Both the ca rbon content and the origin of the organic matter are important for the sor ption. A decrease of carbon content of a soil does not necessarily imply a reduction of sorption capacity for polar organic acids such as MCPA. Nevert heless, our adsorption-desorption experiments suggest that with decreasing carbon content the role of mineral sorption mechanisms could become more pr onounced. Our results showed that interactions of soil organic matter and s oil minerals distinctly influence adsorption properties for MCPA.